/*
 * Copyright 2008-present MongoDB, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.mongodb.scala.internal

import java.util.concurrent.atomic.AtomicBoolean
import org.mongodb.scala.{ Observable, Observer, SingleObservable, Subscription }

import scala.util.{ Failure, Success, Try }

private[scala] case class FoldLeftObservable[T, S](observable: Observable[T], initialValue: S, accumulator: (S, T) => S)
    extends SingleObservable[S] {

  override def subscribe(observer: Observer[_ >: S]): Unit = {
    observable.subscribe(
      SubscriptionCheckingObserver(
        new Observer[T] {

          @volatile
          private var currentValue: S = initialValue

          private val requested = new AtomicBoolean(false)

          override def onError(throwable: Throwable): Unit = observer.onError(throwable)

          override def onSubscribe(subscription: Subscription): Unit = {
            val masterSub = new Subscription() {
              override def isUnsubscribed: Boolean = subscription.isUnsubscribed

              override def request(n: Long): Unit = {
                require(n > 0L, s"Number requested must be greater than zero: $n")
                if (requested.compareAndSet(false, true)) subscription.request(Long.MaxValue)
              }
              override def unsubscribe(): Unit = subscription.unsubscribe()
            }

            observer.onSubscribe(masterSub)
          }

          override def onComplete(): Unit = {
            observer.onNext(currentValue)
            observer.onComplete()
          }

          override def onNext(tResult: T): Unit = {
            Try(accumulator(currentValue, tResult)) match {
              case Success(result)    => currentValue = result;
              case Failure(exception) => onError(exception)
            }
          }
        }
      )
    )
  }
}
